Recently, there has been developed such a mobile digital information system wherein a receiving device mounted in a vehicle receives traffic information (supplied by VICS--Vehicle Information and Communication System) or general information (e.g., news) transmitted as FM-multiplex broadcast from FM broadcasting stations and presents the received information on a screen of a navigator mounted in the vehicle.
There have been used conventional FM-multiplex broadcast receiving devices which search broadcasting stations whose radio-waves have field strengths higher than a certain preset level and automatically selects a suitable one.
The above-mentioned FM-multiplex-broadcast receivers, however, encounter such a problem that traffic information or general information supplied by the VICS utilizing a FM-multiplex broadcast is digitally coded information and, therefore, a received group of data (a packet) may become unusable if it contains an error that can not be corrected. This reduces a correct reception ratio of the broadcast.
Since a FM-multiplex broadcast sends traffic information and general information by using sub-carriers (space) available in the FM broadcast, the reception of the traffic and other general information transmitted by the sub-carrier may sometimes be affected by noise having neighboring frequencies while the main FM broadcast can be received with a sufficient electric field strength.
Consequently, there still remains a problem that traffic and general information supplied by VICS can not sufficiently be received by automatically searching and selecting any one of stations whose broadcasts have an electric field strength higher than a specified level at the receiving place.